1. Field of the Invention
The present invention relates to a method and apparatus for checking the electric continuity of a device made from an electrically conducting material by impedance measurement. Such a device may for example be formed by an assembly of metal parts or else by a monolithic conducting part likely to comprise defects.
2. Description of the Prior Art
The present invention relates to the non destructive testing of such electrically conducting devices and, more particularly, to checking the electric continuity of these devices.
As is known, particularly in the aeronautic field, it is current practice for metal assemblies to be formed by spot connections, for example by rivetting, screwing, welding or similar. Despite these connection methods, by their nature discontinuous, it is often indispensable for the assemblies obtained to provide as uniform a mechanical contact as possible between the assembled parts, so as to provide an electric continuity therebetween which is homogeneous and of low resistance. In fact, it is indispensable for the flow of electric currents passing through these assemblies to take place without privileging certain particular paths, which could generate imbalances prejudicial to the mechanical strength of said assemblies or of the devices associated therewith.
By way of first example, we may consider the action of lightning on an aircraft wing. It is known that the skin of an aircraft wing is formed from metal plates, whose edges overlap and are fixed together by rivetting. In the case where the lightning strikes such as wing, it generates lightning currents flowing therethrough. If the overlapping parts of the metal plates as a whole are not in sufficiently good mechanical contact to provide good homogeneous electric continuity, there then appear, on said wing, zones of good contact and so of low electric resistance and zones of poor contact and so of high electric resistance. The lightning currents then generate, between these zones of different values of electric resistance, electric arcs whose effects may be such as to endanger the safety of the flight of the aircraft.
Similarly, in order to protect certain electronic installations against parasites due to electromagnetic fields, said installations are enclosed in cases capable of forming sealed electromagnetic enclosures. Such cases are generally formed by assembly. Thus, if such an assembly presents a defect of electric continuity in a zone of the case, the currents induced by the parasite electromagnetic fields cannot flow through this zone and so a break in the protection in the electromagnetic enclosure occurs so that the parasites may reach the installation enclosed therein.
Thus, it is often necessary, in spot assemblies, on the one hand, to make sure that the electric continuity is practically as good as that of a monolithic piece and, on the other hand, to check the quality of the electric continuity.
It should be further noted that, for continuous assemblies, for example by weldiog, it may also be useful to check the quality of the electric continuity.
One of the objects of the present invention is therefore, in order to check the quality of the electric continuity of an assembly (a spot or continuous assembly), to measure the low, and even very low contact impedances existing between assembled metal parts.
A method is already known for checking the continuity of metal assemblies, in which a DC measuring current is injected between the assembled parts and the resulting DC voltage is measured therebetween. Consequently, it is possible to know the electric resistance between said parts. However, as will be shown hereafter, there exist numerous cases in which such a method cannot provide a significant measurement.
The present invention overcomes this drawback. It allows the electric resistivity and the skin effect of a monolithic conducting piece to be measured.TM.with the aim for example of detecting defects.